A molten carbonate fuel cell typically comprises a stack of individual cells or cell units. Each individual cell comprises a cathode and an anode, with their associated current collectors, and a matrix saturated with the molten electrolyte arranged between the two respective electrodes. A fusible fluid alkali carbonate mixture is typically used as the electrolyte. For this reason, the operating temperature of the fuel cell is in the range from 500.degree. C. to 700.degree. C. The alkali carbonate mixture may comprise lithium carbonate and potassium carbonate mixtures, lithium carbonate and sodium carbonate mixtures, or a ternary mixture including lithium carbonate, potassium carbonate, and sodium carbonate. All three of these different mixtures share in common, that lithium carbonate is an absolutely necessary or essential component of the respective mixture.
The individual cells are respectively separated from one another by a gas-tight separator. Herein, the term "gas-tight" means "not gas permeable". According to one variation, the separator comprises a flat planar plate, while the gas permeable cathode current collector has a wavy corrugated configuration and is arranged between the cathode and the separator plate, and the anode current collector similarly has a wavy corrugated configuration and is arranged between the anode and the respective separator plate. This construction forms a respective first gas supply passage for supplying air or some other oxygen-containing gas to the cathode, and a respective second gas supply passage for supplying hydrogen or other fuel gases to the anode. According to another variation, the wavy corrugated current collectors are gas-tight, so that they simultaneously form the necessary separators.
Thus, the respective current collectors serve dual purposes. First, the current collectors conduct and carry the electrochemically generated current away from or to the respective cathode or anode. Secondly, the current collectors form the passages or spaces necessary for supplying the reaction gases to the cathode or the anode respectively.
In order to mechanically strengthen the cathode, it is possible to additionally provide a gas permeable, electrically conducting, flat planar metal support plate between the cathode and the wavy corrugated cathode current collector or the wavy corrugated separator plate. The support plate may, for example, be embodied as a perforated metal plate or sheet.
Due to the supply of air or other oxygen-containing gas, and the high operating temperature, the current collector provided for the cathode is subject to severe corrosion effects. In order to resist the corrosive attack, the respective current collector and the other components located in the cathodic half space in the fuel cell are made of a high alloy stainless steel, which contains at least 16 wt. % of chromium. Nonetheless, it has been found in practice that oxide layers form on the surfaces of these components during operation of the fuel cell. While these oxide layers are mostly electrically well conducting, they lead to a loss of the electrolyte however. Namely, on the one hand, the iron oxide, chromium oxide, or other metal oxide of the oxide layer reacts with the lithium ions of the electrolyte while forming lithium ferrite, lithium chromite, or the like, and on the other hand, the potassium or sodium ions of the electrolyte reacting with the chromium oxide of the oxide layer form potassium chromate and/or sodium chromate. In this manner, the electrolyte is chemically broken down and lost, which leads to a degradation of the cell and therewith a decrease in the cell output power.